Rotor brakes are used especially in rotary-wing aircraft such as, for example, helicopters, and in some types of helicopters, they are a safety-relevant system. They serve to brake the rotation of a rotor, especially of a tail rotor, when the rotary-wing aircraft or helicopter is on the ground. Rotor brakes are therefore usually arranged at an outlet of a gear of the rotary-wing aircraft where there is a connection to a tail rotor shaft. As a rule, rotor brakes are configured as disk brakes. The braking force here is exerted using calipers, brake linings or pressure disks by mechanical or hydraulic means. In the case of hydraulic brakes, the hydraulic brake system can be coupled either to an independent system or else to the hydraulic system of the rotary-wing aircraft. Occasionally, rotor brakes that are configured as drum brakes are also used. Rotor brakes for rotary-wing aircraft may only be deployed on the ground and only once the (main) rotor has reached a certain speed, which is usually 40% to 50% of the rated rotor speed. Such a limitation is meaningful since, due to aerodynamic forces, the rotor speed drops relatively quickly to about 40% to 50% of the rated rotor speed, but after that, it decreases only relatively slowly. Dimensioning rotor brakes that are already effective at 70% to 100% of the rated rotor speed would entail very strong brakes, a substantial weight of the rotor brake as well as increased manufacturing costs. Therefore, especially in the civilian sector, such a design is neither desired nor, as a rule, necessary.
With conventional rotor brake systems, actuating the rotor brake above a value of 40% to 50% of the rated rotor speed can lead to overheating of the brake disk or of the brake drum since, in this speed range, excessive braking energy is applied to the brake system. In the extreme case, the structure of safety-relevant brake components such as, for example, the brake disk or the brake drum, can be changed and fail. As an example, mention should be made of the fact that an actuation of the rotor brake at 70% rather than at 50% of the rated rotor speed means that approximately twice as much braking energy has to be absorbed by the rotor brake system.
Moreover, an operating error can easily occur with conventional rotor brakes. For example, it is possible for the pilot to accidentally actuate the rotor brake during flight, which usually has fatal consequences. Or the rotor brake can be actuated on the ground before the permissible speed of approximately 40% to 50% of the rated rotor speed has been reached which, as already mentioned, might in certain cases cause a failure or a safety-critical change in the brake system. With hydraulically activated brakes, an erroneous actuation of the braking function can also occur due to a temperature-related increase in the pressure of the hydraulic system or a malfunction of the hydraulic system.
Statutory or national regulations, for example, according to JAR/FAR, require that a display has to indicate any braking force that is exerted on a rotor brake during flight. Such an indication, however, does not prevent the rotor brake from being actuated during the flight or above a permissible rotor speed.
European Patent Document EP 0 894 712 B1 discloses a rotor brake with a system that is supposed to prevent an erroneous actuation of the rotor brake system. The rotor brake comprises a brake disk as well as a brake actuator with a caliper that interacts with the brake disk. The caliper is configured so as to pivot or move with respect to the brake disk. When the rotor is operating at the rated speed or above a permissible rotor speed, the caliper is pivoted away from the brake disk and consequently, even in case of a malfunction or an operating error, it cannot act upon the brake disk and trigger a braking effect. In contrast, when the rotor reaches an appropriately low, permissible rotor speed on the ground, then the caliper is pivoted towards the brake disk and, if actuated, can exert a braking effect on the brake disk. This prior art system is relatively complex in terms of design, it is expensive and heavy, which runs counter to the requirements made of lightweight design in aviation technology.
German Patent Document DE 195 00 539 A1 describes a rotor brake that comprises a braking force absorption element that can be non-rotatably connected to a rotor shaft as well as a stationary brake actuator for actuating a braking force transmission element. The braking force transmission element can be functionally coupled to the braking force absorption element in order to exert a braking effect. The rotor brake has a rotor brake deactivation device having a clutch that can be disengaged into a free-wheeling position when the rotor brake is not in use, especially during flight, and said clutch can be meshed in a form-fitting engagement with a hub of the rotor brake when the rotor brake is to be used.